There have been many studies regarding low-temperature soluble (lower critical solution temperature-type: LCST-type) thermoresponsive polymer compounds in an aqueous solution. Due to the revelation of the phase-transition mechanism, molecular designing to control the phase transition temperature has also been clarified. For example, low-temperature soluble polymer compounds such as poly(N-isopropylacrylamide) are used as a basic material for cell sheet technology applicable to cell separation (Non-patent literature 1) or drug delivery systems (Non-patent literature 2), and serve as a trigger to bring new trends to regenerative medicine.
In contrast, even though the development of positive (upper critical solution temperature-type: UCST-type) thermoresponsive polymer compounds in an aqueous solution or under physiological conditions would result in basic materials that would greatly impact the engineering field, very few studies exist. This is because there are only a few examples of polymer compounds that exhibit UCST behavior in aqueous systems (Non-patent literature 3 to 5), and most of them do not exhibit thermoresponsive properties under physiological conditions, i.e., physiological pH, salt concentration and temperature.